There are over 18,000 hospitalizations and 75,000 hospitalization days annually for children suffering vaso-occlusive crises secondary to sickle cell disease. The vast majority of these hospitalizations are for pain crisis, making pain one of the most significant morbidities for children with sickle cell disease. The acute treatment for these pain crises has changed little over the past three decades, mainly relying on intravenous fluids, opioids and non-steroidals. More recently, there has been a greater recognition of the role that nitric oxide plays in the pathogenesis of pain crises, and inhaled nitric oxide is being studied as a possible treatment. We have recently tested an alternative approach, intravenous magnesium, that produces vaso- dilation through both endothelium-dependent nitric oxide mechanisms, as well as direct action on the underlying vascular smooth muscle. The safety profile, ease of administration, and low cost of magnesium make its potential as a sickle cell therapy particularly exciting. In this application, we utilize the Pediatric Emergency Care Applied Research Network to develop a four-center collaboration between pediatric emergency medicine and pediatric sickle cell centers to test this novel therapy through a double-blind, randomized, placebo controlled trial assessing the impact of the addition of intravenous magnesium to standard therapy for sickle cell pain crisis. The primary clinical outcome is the decrease in length of hospital stay for pediatric sickle cell pain crisis, with a secondary clinical outcome measuring the effect of intravenous magnesium on the use of opioid pain medication during the hospitalization. In addition to the clinical outcomes, we will gain valuable insight into the pathophysiology of sickle cell crisis through the measurement of hemolysis, nitric oxide pathway metabolites, and markers of cellular injury and inflammation. At the conclusion of this study, we have the potential to improve the care of children with sickle cell disease using a low cost, low risk treatment.